1. Field of the Invention
This invention relates generally to direct access storage devices and, more particularly, to control of arm movement in disk drive devices.
2. Description of the Related Art
In a conventional computer data storage system having a rotating storage medium, such as a magnetic or magneto-optical disk, data is stored in a series of concentric or spiral tracks across the surface of the disk. A magnetic disk, for example, can comprise a disk substrate having a surface on which a magnetic material is deposited. The digital data stored on a disk is represented as a series of variations in magnetic orientation of the disk magnetic material. The variations in magnetic orientation, generally comprising reversals of magnetic flux, represent binary digits of ones and zeroes that in turn represent data. The binary digits must be read from and recorded onto the disk surface. A read/write head produces and detects variations in magnetic orientation of the magnetic material as the disk rotates relative to the head.
Conventionally, the read/write head is mounted on a disk arm that is moved across the disk by a servo. A disk drive servo control system controls movement of the disk arm across the surface of the disk to move the read/write head from data track to data track and, once over a selected track, to maintain the head in a path centered over the selected track. Maintaining the head centered over a track facilitates accurate reading and recording of data. Positioning read/write heads is one of the most critical aspects of recording and retrieving data in disk storage systems. With the very high track density of current disk drives, even the smallest head positioning error can potentially cause a loss of data that a disk drive customer wants to record or read. Accordingly, a great deal of effort is devoted to servo control systems.
A servo control system generally moves a read/write head to a desired track and maintains the head in a position centered over that track by reading servo information recorded on the disk surface. The servo information comprises track identification information and a position-encoded servo pattern of high frequency magnetic flux transitions, generally flux reversals, that are pre-recorded in disk servo tracks. The flux transitions are recorded as periodic servo pattern bursts formed as parallel stripes in the servo tracks. When the read/write head passes over the servo pattern flux transitions, the head generates an analog signal whose repeating cyclic variations can be demodulated and decoded to indicate the position of the head over the disk. The position indicating information can be used to produce a corrective signal that is referred to as a position error sensing (PES) signal. The PES signal indicates which direction the head should be moved to remain centered over a selected track and properly read and write data.
In the sector servo method of providing servo track information, each disk surface is divided into angularly-spaced sectors, with each sector containing both pre-recorded servo track information and customer data. Typically, the tracks on a sector servo disk are partitioned by having a short servo track information area followed by a customer data area. FIG. 1 illustrates a portion of a track 102 from a conventional sector servo disk, showing a servo track information area and the customer data area that follows. The servo track information area typically includes a servo mark field 104 that indicates servo information follows in the track, and also serves an automatic gain control (AGC) function. The servo mark is followed by a sector identification field 106, and then a gray code field 108 that provides track number information. Next, a servo pattern field 110 contains a servo burst pattern. A synchronization field 112 then immediately precedes a customer data field 114, where disk users read and write their data. The servo read head is typically the same head used for reading the customer data.
In the FIG. 1 illustration, the servo burst pattern 110 is indicated as a quadrature pattern having four servo bursts labeled A, B, C, and D. Those skilled in the art will understand that the servo bursts will, when decoded, produce the PES signal described above. After a seek operation to move the read/write head to a desired track, a conventional digital servo control system generates a write inhibit signal that will prevent any data recording if the read/write head is not at the proper track and if it is away from the track centerline by more than a threshold limit. The servo control system typically achieves the write inhibit by shutting off the write gate of a write control processor, thereby preventing off-track write operations.
A conventional servo control system determines when the write inhibit signal should be produced by performing a series of steps that are implemented in software. First, the servo controller receives the servo track information, including the PES servo pattern burst signal, and calculates the distance of the read/write head from the track centerline. If the head is not at the desired track, no write occurs. If the head is offset from the track centerline by more than a predetermined threshold, the controller shuts off the write gate, preventing any write operations. Unfortunately, such software processing necessarily creates a delay in actually posting the write inhibit. Several operating cycles of the servo controller central processor clock may pass before the readback signal is demodulated, the off-track distance is calculated, and the distance is compared to the predetermined threshold to make the write inhibit decision. That is, if the read/write head is off-track, this fact is not detected until the next sector servo interval, after the servo burst has been transduced, the quadrature pattern has been demodulated, and the off-track distance determined.
The time delay imposed by the software-determined write inhibit scheme has two bad effects. First, the read/write head will likely be moved even farther away from the track center while the servo information is being processed and the write inhibit decision is being made. Second, the write inhibit decision from a prior sector may persist into the next following sector, so that the write inhibit will last too long. If a write inhibit decision could be processed and implemented during the same sector for which position information is transduced, an off-track excursion could be detected quickly after the head position moved out of tolerance, and a write inhibit decision could be cut off quickly after it is no longer needed.
From the discussion above, it should be apparent that there is a need for a disk drive servo control system that can provide write inhibit decision making without software processing delays, can produce a write inhibit decision quickly after an off-track excursion, and can terminate a write inhibit decision quickly after it is no longer needed. The present invention fulfills this need.